1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device and, in particular, a semiconductor device having an unhomogeneous distribution of impurity concentration.
2. Description of the Prior Art
It is well known that silicon varactor diodes having a hyper-abrupt junction may be easily manufactured by using an alloying diffusion technique. In the case of a varactor diode made with a compound semiconductor such as GaAs, however, it is not possible to perform a satisfactory diffusion of, for example, the donor atoms. This in turn makes it difficult to obtain a varactor diode having a large variation of capacitance relative to the applied voltage. In the past, attempts have been made to produce hyper-abrupt junctions in GaAs varactor diodes by using either an ion-implantation technique or a molecular beam epitaxial growth technique. Use of each of these techniques, however, entails certain disadvantages. In the case of the former technique, it is difficult to carry out the step of annealing which follows ion-implanation and in the case of the latter technique, difficulty is encountered during the doping process.
Japanese Patent Preliminary Publication No. Sho 52-128057 discloses a method including the ion-implantation of sulfur (S), which utilizes the abnormal diffusion phenomenon of sulfur (S) to obtain an impurity concentration distribution, with an impurity level variation ranging from 1.5 to 4. According to this method, an insulating film which serves as the protective film is deposited onto the opposing surfaces of the GaAs wafer during the annealing step which follows the ion-implantation of sulfur (S). The method described by this publication, however, is accompanied by a significant drawback in that it is not possible to eliminate the abnormal diffusion of sulfur (S) at the time of annealing.